1. Field of the Invention
The present invention relates to a method of setting a link for eliminating erroneous map matching in a map matching system that identifies the road on which a vehicle is traveling based on current location information acquired by on-board equipment mounted on the vehicle.
2. Description of Related Art
Conventionally, map matching processing, which processes the information from various sensors to estimate and match the current location of a traveling vehicle on a road on a map, is performed by a car navigation system or at an information center which receives sensor information through telecommunication, and the result of which has been utilized for route guidance, traffic information provision, and so on. However, there have been cases in which incorrect map matching takes place in areas where there are multiple paralleling roads with a different road width or where a road network is complicated.
That is, when the on-board equipment to be mounted on the vehicle is a navigation system, the current position of the vehicle is calculated from the moving direction of the vehicle measured by a directional sensor such as a gyroscope, and the travel distance measured by a speed sensor or a distance sensor.
In addition, the information from GPS is also utilized to obtain an accurate current location thereby identifying the location of the traveling vehicle on a road so that a mark indicating the current location is displayed on a road on a map of the car navigation system.
A technique that calculates a current location of a vehicle and corrects the calculated location of the vehicle by identifying the road on which the vehicle is traveling is called a map matching technique.
When the on-board equipment is not a car navigation system but consists of a GPS alone without autonomous navigation, the information from which the current location of the vehicle is calculated is that of the GPS only, and it is difficult to accurately calculate the current location of the vehicle due to large position error of the GPS.
And when the on-board equipment consists of a GPS alone, GPS information is likely to be transmitted through telecommunication to an information center where an estimated vehicle location on a road on a map is calculated.
Whether the on-board equipment consists of a car navigation system or of a GPS alone, the map matching technique to estimate the current location of a vehicle on a road is generally performed as follows.
When a current location of the vehicle estimated based on the information from various sensors including a GPS is not on a road on a map, the modified location is decided as follows: a perpendicular is drawn from the current location toward a road near the current location and the intersection of the two lines is the modified location. A road on a map is represented as a line segment called a link that has no width as described below.
That is, a road on a map is represented as line segments, for example, divided at a crossroad. A line segment is generally called a “link”, each of which is given a link number. A road is identified either by setting a link for each direction of a road corresponding to the traveling direction of the vehicle, or setting a single link for both directions of the road and adding a traveling direction as associated information. A link that has no width is set at the center of the road.
In an area where there are paralleling roads, i.e. where links are aligned in parallel to each other, there has been risk that an adjacent road different from the actual road is map-matched due to the measurement error of the current location. In particular, when a wide road and a narrow road are aligned in parallel, it has been highly likely that even though the vehicle is running on the wide road, the location of the vehicle is projected closer to the link for the narrow road than to the link for the wide road, and thus the location of the vehicle is map-matched onto the link of the narrow road.
Thus, there has been risk that erroneous information is generated when a wrong road is map-matched and based on that information road traffic information is generated at an information center. A car navigation system could also give a driver incorrect road guidance.
To solve this problem, Japanese Patent No. 3596939 (document 1) describes a method for avoiding erroneous map matching by setting a threshold for the distance (length of the perpendicular) from the current location of a vehicle to a candidate road nearby to be selected in accordance with road width thus taking into consideration the influence of the road width.
Further, Japanese Patent Application Laid-Open No. 2004-226341(document 2) describes a method of preventing erroneous map matching for a vehicle traveling on a lane, which is closer to another paralleling road, of a wide road having multiple lanes by setting a virtual link for each lane of the road.
However, those techniques have had following problems.
There has been a problem with document 1 in that satisfactory map matching may still not be achieved, and document 2 in that setting multiple virtual links for a road requires several times more processing time for map matching.
Document 2 also describes a method in which only one virtual link is set thus providing one virtual link for one road. However, when there are three paralleling roads, the middle road has two outer virtual links, thereby still affecting processing time. There has been also a problem that when a vehicle is map-matched only to the outer virtual links, the data for a vehicle traveling near the center of a road may not be matched correctly if the road is broad. There has been also a problem in that when the outer virtual link is set based on the road width alone, a multi-layered road may be given wrong links, thereby causing a problem in map matching.